Conventional cooling means involving soldering the component to a copper track, itself mounted on an electrically insulating ceramic of the AlN type or the like serving to insulate the high potential track and the component from the soleplate and grounded cooling device.
The copper-track-and-AlN multilayer structure is well suited to electrically insulating such a power component. However, the numerous interfaces, soldering, glues, and greases used to manufacture the multilayer structure constitute a considerable thermal resistance (about 15.times.10.sup.-6 kelvins per watt (K/W) for 625 .mu.m of AlN and 300 .mu.m of Cu) affecting the capabilities of the structure to remove suitably the Joule heat losses from the power components, by itself or via a cooling device.
Furthermore, the more the potential is increased, the thicker the dielectric ceramic layer must be, whereas it is known that ceramic dielectrics are not good conductors of heat.
It is known that artificial diamond may be used, offering the advantage of being an excellent electrical insulator, and of having thermal conductivity approximately 10 times greater than AlN (diamond: 1,500 watts per meter kelvin (W/m.K); AlN: 180 W/m.K).
Bonding an artificial-diamond substrate to a copper or aluminum track generally involves:
a PVD metal-plating first step during which a fine layer of titanium, of platinum, and of gold is deposited on the artificial-diamond substrate; and PA1 a soldering second step in which conventional soldering methods are used to solder the diamond/Ti,Pl,Au composite substrate to the copper or aluminum track. PA1 at least one of the faces of the diamond substrate is covered with a piece of aluminum foil; PA1 said diamond substrate and said piece of aluminum foil are disposed in a chamber under a controlled atmosphere, and they are bonded together by a first thermocompression step under Argon or under a vacuum, thereby forming a composite multilayer substrate; PA1 said composite multilayer substrate is disposed on a metal substrate, the aluminum surface being in contact with the metal substrate; and PA1 said composite multilayer substrate and said metal substrate are bonded together by performing a second thermocompression step.
The heat dissipation gain is about 25% to 30% compared with the AlN multilayer structure.
That gain is small compared with what might be expected in view of the heat-conducting properties of artificial diamond.
This is due to the soldering which creates considerable thermal resistance (total thermal resistance of about 13.times.10.sup.-6 K/W for 300 .mu.m of diamond, 100 .mu.m of solder, and 300 .mu.m of Cu, including 12.times.10.sup.-6 K/W induced by the solder and the interfaces).
Furthermore, the very high purchase price of artificial diamond makes the ratio between production cost and heat dissipation gain much higher than with the AlN technique and therefore prohibitive for mass production purposes.